Uniformly tapered transition mandrel



Nov. 28, 1967 1 P. A. SZENTE 3,354,695

UNIFORMLY TAPERED TRANSITION MANDREL Filed July 16, 1964 L 2 INVENTOR P5020 ASZENTE A TTORNE Y United States Patent 3,354,695 UNIFORMLY TAPERED TRANSITION MANDREL Pedro A. Szeute, Sao Jose dos Campos, Sao Paulo, Brazil, assignor to the United States of America as represented by the United States Atomic Energy Commission Filed July 16, 1964, Ser. No. 383,263 2 Claims. (Cl. 72-476) ABSTRACT OF THE DISCLOSURE A three-piece mandrel for forming a transition waveguide section comprises alternate cylindrical fiat and conical surface sections that taper with optimum continuity from a rectangular to circular cross section.

The invention disclosed herein was made under, or in, the course of Contract No. AT(043)-21 with the United States Atomic Energy Commission.

The present invention relates generally to a metal forming die and in particular to a mandrel for forming a uniformly tapered transition from a preselected length of rectangular, microwave waveguide, and the method of making such mandrel.

Previous methods and devices for shaping waveguides, and in particular, for changing the cross section of a Waveguide from a rectangular shape to a circular shape generally involves the application of at least two operations. Furthermore the application of at least two operations in many cases results in a waveguide transition which fails to produce the surface continuity not only desired but also essential for optimum microwave power transmission therethrough,

The present invention overcomes the above-mentioned shortcomings by providing a unique mandrel which will form, in one operation, a waveguide transition which progresses smoothly along its length from a rectangular cross section to a circular cross section.

a microwave waveguide mandrel capable of repeatedly forming, with optimum continuity along the transitional region, a waveguide transition having a rectangular to circular cross section.

It is yet another object of the present invention to provide a method of constructing a mandrel for use in forming a transition within a waveguide wherein the surface of the mandrel varies progressively from'rectangular'to circular shape. i

It is still another object of the present invention to provide a simple, durable and inexpensive mandrel far use in forming a transition in a waveguide from a rectangular to a circular shape, which mandrel can be manufactured -simply and inexpensively with a minimum of machining equipment.

Other objects and advantages will be apparent in the following description and claims considered together with the accompanying drawings, in which FIGURE 1 is a plan view of an assembled, multipart mandrel of the present invention.

FIGURE 2 is an elevation view of the invention of FIG. 1.

FIGURE 3 is an end view of the invention of FIG. 1.

FIGURE 4 is an end view of the opposite end of the mandrel to that shown in FIG. 3.

FIGURE 5 is a perspective view of a central member of a multipart mandrel in accordance with the invention.

FIGURE 6 is a plan view exemplifying the construction method of portions of the present invention, indicating in phantom line the final profile of two particular side members of the invention.

FIGURE 7 is an end view of the mandrel construction as depicted in FIG. 6.

In brief, a uniformly tapered, transition element, termed transition hereinafter, as produced utilizing the apparatus of the invention, is generally utilized as a means for coupling a rectangular waveguide to another waveguide of circular and larger total area cross section, As such, the transition includes a rectangular cross section terminus and a circular cross section terminus joined by a central tubular portion proportioned and dimensioned to provide microwave transmission characteristics appropriate to effect the transition with minimized energy loss and disturbance.

More particularly, referring to FIGS. 1-4, there is shown a mandrel 10 used in accordance with the invention for forming waveguide transitions and comprising essentially a central member 12 and identical side members 14 and 16 afiixed thereto having an external configuration corresponding to the interior of the transition described above and provided as set forth hereinafter. The

' central member 12 is essentially a rectangular, flat portion, wherein the lateral edges are uniformly rounded, and wherein such rounded edges taper gradually a few degrees along the length of the member 12. Member 12 has, in essence, the configuration of a central slab portion symmetrically removed circumjacent about, and along, the axis of an elongated truncated cone. The identical side members 14 and 16 have, in general, the shape of the outer remaining portions of such a truncated cone with a central slab removed, wherein however, the thickness of the central slab which has been removed is made equal to the diameter of the truncated upper base of the cone, and wherein the taper of the cone is much more pronounced.

An axially extending central hub 18 having an axial threaded bore 20 formed therein is integrally formed onto the end of the mandrel 10, and in particular, coaxially formed on the tapered end of central member 12. Such internally threaded, central hub 18 provides means for gripping the mandrel 10 at such time as the mandrel is utilized in the formation of a waveguide transition. An axial threaded bore 22 is provided on the end of the central member 12 opposite the end thereof having the central hub 18. The threaded bore 22 is likewise utilized as a means for gripping the mandrel 10 in order to withdraw same from the waveguide after forming the transition. As shown in FIGS. 1' and 2 the side members 14, 16 are secured to the central member 12 by respective sets of set screws 24, 26 respectively, wherein the heads of the set screws 24, 26 are recessed within the side members 14, 16 respectively to avoid any interruption of the smooth outer surface thereof.

A transition is formed utilizing the mandrel of the present invention by introducing the mandrel 10 at its generally rectangular end into a preselected, rectangular Waveguide section of suitable cross section and length, and which is heated to the required temperature to allow ready deformation thereof. The mandrel 10 is gripped about the internally threaded, central hub 18 and is forced into the heated portion of waveguide until its entire length is within the waveguide, and particularly, until the round end of the mandrel is within the Waveguide section. Thus, there is formed in oneoperation, a waveguide transition which progresses smoothly along its length from a rectangular cross section to a circular cross section. With soft and ductile materials heating may not be re quired.

In forming the transition from a length of rectangular waveguide, there is an allowable limit of expansion to beexpected regarding the deformation of the rectangular waveguide dimensions, For example, excellent transitions have been made when less than 20% expansions of the orossrsectional area of the rectangular waveguide were maintained. However, even larger expansions are allowable and may be experienced, depending upon the ductility and deformability of the material of which the rectangular waveguide is made, without impairing the properties of the transition made utilizing the present invention. In any case, care should be taken to avoid expansion of the material to the point that it becomes brittle. Such a condition results in a finished piece that is susceptible to crackings and consequent leakage during use. Assuming a 20% expansion the following equation sets the limits of the dimensions of the formed transition:

wherein A is the diameter across the smaller end of the central member 12, and C is the constant thickness of the central member 12. The equation states in essence that the round end should have a circumference less than 1.2 times the perimeter of the rectangular end, but should be more than the perimeter of the rectangular end.

The cross-sectional dimensions of the waveguide, ie both the original rectangular dimensions and the diameter of the circular cross section following deformation, are chosen commensurate with the particular propagation frequency (or band of propagation frequencies) at which the waveguide is to operate. The rate of change of the cross-sectional dimensions with respect to the axial dimension thereof, should be sufiiciently gradual to discourage the propagation of undesired waveguide modes which are sometimes produced by too abrupt a transition of the waveguide cross section. The maximum length of taper is limited not so much by the operating parameters thereof, but instead, is decreed by the obvious limitations of practical lengths and the cost of constructing the transition. That is, a transition formed by means of the present in vention can be of any desired length greater than a length of at least one-half wavelength of the propagated wave.

Construction of the mandrel 10. in accordance with the invention is exemplified by means of FIGS. 5, 6 and 7. The center member 12 of the mandrel is formed of a rectangular stainless steel bar of a constant thickness, C, wherein however, the width tapers slightly from the larger end having a width D, to a smaller end having a width A, Particularly, the edges of member 12 are rounded as shown in FIGURES 3 and 4 such that the dimension D actually defines the diameter of the larger end Of an axially extending portion of a tapered cylinder, and thus is the diameter of the arcuate surfaces of the central member 12. Likewise, dimension A defines the diameter of the same axially extending portion of the tapered cylinder at its smaller end. Thus, it may be seen that member 12 comprises in general a flat rectangular solid, wherein the width is slightly tapered, and the thickness is defined by rounded, arcuate surfaces.

The side members 14 and 16 are machined from a parallelopiped 27 formed of three rectangular, flat, stainless steel bars bolted together as shown in FIG. 6. A center bar 28 isv placed between outer bars 30 and 32, wherein the dimension C of the center bar equals the dimension C of the central member 12 of FIG. 1, and the base dimensions of the assembled parallelopiped 27 are equal to, or greater than dimension D; The parallelopiped 27 is then machined to form a truncated cone of base diameter D and upper diameterC. Upon completion of the machining,

as defined by the plantom lines on FIG. 6, the parallelopiped 27 is disassembled and the center bar 28 is discarded; the two finished side members 14 and 16 are retained for subsequent assembly to the central member 12 as shown on FIGS. 1-4 by means of set screws or bolts 24 and 26 respectively, which are passed through the same holes originally used to secure the outer bars 30, 32 to the center bar 28 in the parallelopiped 27 before machining thereof.

A number of the mandrels in accordance with the invention have been constructed, one of such constructions utilized a central member 12 of length 7.92 inches wherein D equals 3 inches, A equals 2.84 inches, and C equals 1.34 inches. The transverse dimensions were chosen to conform to the equation hereinabove disclosed. The length of the mandrel was chosen for optimum transmission of radio-frequency power having a frequency of 2857 megacycles per second by making its length greater than one half the wavelength of a 2857 megacycle wave. The operating characteristics of the transition formed by the invention are such as to allow, however, optimum power transmission of a frequency bandwidth equal to at least 10% of the design frequency of 2857 megacycles per second.

While the invention has been disclosed with respect to a single preferred embodiment, it will be apparent to those skilled in the art, that numerousvariations and modifications may be made within the spirit and scope of the invention. For example, although the simplest and thus preferred construction of the mandrel utilizes three pre-formed members demountably secured together as hereinbefore described, the mandrel could be machined from a single block of material of suitable dimensions, i.e., the mandrel configuration shown in FIGURES 1-4 could be shaped from a solid block of material by extensive machining operations. Thus it is not intended to limit the invention except as defined in the following claims.

What is claimed is: 1. A mandrel for forming from a rectangular length of waveguide in one operation a microwave waveguide transition from a rectangular waveguide to circular waveguide capable of transmitting therethrough with minimum power losses microwave power of preselected wavelength said mandrel comprising (a) a body member having a length equal to at least a half wavelength of said preselected wavelength;

('b) said body member having a first terminus of substantially rectangular cross section and dimensions to facilitate snug insertion of such end into one end of said length of rectangular waveguide;

(c) said body member having a second opposite terminus of circular and larger total area cross section;

(d) said body' member having a central portion extending between said first terminus and said second terminus, said central portion having a geometrically progressing cross section therealong defined by a generally rectangular shape having rounded lateral edges along the entire length thereof, said body portion having side portions adjacent each of the longer dimensions of said central portion, said side portions having cross sections along the entire length thereof which conform to a segment on a chord of a circle coaxial with the central aXis of said central portion, the areas of said cross sections continuously increasing beginning at said first terminus and progressing toward said second terminus.

2. A mandrel according to claim 1, wherein:

said central portion of said body member is a bar having a constant thickness, a gradually tapering width, and a length equal to at least one half the. wavelength of said preselected Wavelength;

said side portions of said bar are first and second side members in the shape of hyperbolic conical segments integrally secured on opposite surfaces of said bar, and extending substantially the entire length thereof;

the outer surfaces of each of said side members is rounded such that the bases thereof at said second terminus have a curvature of a diameter equal to the inner diameter of said circular waveguide;

the width of each of said side members tapers to a conical hyperbolic apex at said first terminus;

the width of said side members near said first terminus is substantially less than the width of said bar thereat; and

said side members and said bar, upon being assembled, define a substantially rectangular cross section at said first terminus and further define a circular base having a diameter equal to the inner diameter of said circular waveguide at said second terminus.

References Cited UNITED STATES PATENTS 437,840 10/1890 Van Haagen 72479 5 1,912,600 6/1933 Spatta 72358 2,464,323 3/ 1949 Lee 72479 2,485,534 10/1949 Mayne et a1. 72370 FOREIGN PATENTS 10 158,957 2/ 1921 Great Britain.

18,466 11/ 1956 Germany. 546,794 4/ 195 6 Belgium.

RICHARD J. HERBST, Primary Examiner. 

1. A MANDREL FOR FORMING FROM A RECTANGULAR LENGTH OF WAVEGUIDE IN ONE OPERATION A MICROWAVE WAVEGUIDE TRANSITION FROM A RECTANGULAR WAVEGUIDE TO CIRCULAR WAVEGUIDE CAPABLE OF TRANSMITTING THERETHROUGH WITH MINIMUM POWER LOSSES MICROWAVE POWER OF PRESELECTED WAVELENGTH SAID MANDREL COMPRISING (A) A BODY MEMBER HAVING A LENGTH EQUAL TO AT LEAST A HALF WAVELENGTH OF SAID PRESELECTED WAVELENGTH; (B) SAID BODY MEMBER HAVING A FIRST TERMINUS OF SUBSTANTIALLY RECTANGULAR CROSS SECTION AND DIMENSIONS TO FACILITATE SNUG INSERTION OF SUCH END INTO ONE END OF SAID LENGTH OF RECTANGULAR WAVEGUIDE; (C) SAID BODY MEMBER HAVING A SECOND OPPOSITE TERMINUS OF CIRCULAR AND LARGER TOTAL AREA CROSS SECTION; (D) SAID BODY MEMBER HAVING A CENTRAL PORTION EXTENDING BETWEEN SAID FIRST TERMINUS AND SAID SECOND TERMINUS, SAID CENTRAL PORTION HAVING A GEOMETRICALLY 